15-441: Computer Networking
Lecture 3: Design Philosophy &
Applications
Copyright © CMU, 2007-2010.
Lecture Overview
• Last time:
• Protocol stacks and layering
• OSI and TCP/IP models
• Application requirements
• Application examples
• ftp
• http
• Internet Architecture & Performance intro
S’ 10
Lecture 3: Applications
2
Applications and Application-Layer
Protocols
• Application: communicating,
distributed processes
• Running in network hosts in
“user space”
• Exchange messages to
implement app
• e.g., email, file transfer, the
Web
application
transport
network
data link
physical
• Application-layer protocols
• One “piece” of an app
• Define messages exchanged
by apps and actions taken
• User services provided by
lower layer protocols
S’ 10
application
transport
network
data link
physical
Lecture 3: Applications
application
transport
network
data link
physical
3
Client-Server Paradigm
Typical network app has two pieces: client and server
Client:
• Initiates contact with server
(“speaks first”)
• Typically requests service from
server,
• For Web, client is implemented in
browser; for e-mail, in mail
reader
application
transport
network
data link
physical
request
Server:
• Provides requested service to
client
• e.g., Web server sends
requested Web page, mail server
delivers
e-mail
S’ 10
Lecture 3: Applications
reply
application
transport
network
data link
physical
4
Server and Client
Server and Client exchange messages over
the network through a common Socket API
Clients
Server
TCP/UDP
user
space
ports
TCP/UDP
Socket API
IP
IP
Ethernet Adapter
Ethernet Adapter
S’ 10
Lecture 3: Applications
kernel
space
hardware
5
Network Addressing Analogy
Telephone Call
Network Programming
Professors at CMU
412-268-8000
ext.123
S’ 10
Applications/Servers
412-268-8000
ext.654
Web
Port 80
Mail
Port 25
Extension
Port No.
Telephone No
Central Number
Exchange
Area Code
IP Address
Network No.
Host Number
15-441 Students
Clients
Lecture 3: Applications
6
What Service Does an
Application Need?
Data loss
Timing
• Some apps (e.g., audio) can
tolerate some loss
• Other apps (e.g., file transfer,
telnet) require 100% reliable
data transfer
• Some apps (e.g., Internet
telephony, interactive
games) require low delay to
be “effective”
Bandwidth
• Some apps (e.g., multimedia) require minimum amount of
bandwidth to be “effective”
• Other apps (“elastic apps”) make use of whatever bandwidth they
get
S’ 10
Lecture 3: Applications
7
Transport Service Requirements
of Common Apps
Application
file transfer
e-mail
web documents
real-time audio/
video
stored audio/video
interactive games
financial apps
S’ 10
Data loss
Bandwidth
Time Sensitive
no loss
no loss
no loss
loss-tolerant
elastic
elastic
elastic
audio: 5Kb-1Mb
video:10Kb-5Mb
same as above
few Kbps
elastic
no
no
no
yes, 100’s msec
loss-tolerant
loss-tolerant
no loss
Lecture 3: Applications
yes, few secs
yes, 100’s msec
yes and no
8
Other Requirements
• Network reliability
• Network service must always be available
• Security: privacy, denial of service,
authentication, …
• Scalability.
• Scale to large numbers of users, traffic flows, …
• Manageability: monitoring, control, …
S’ 10
Lecture 3: Applications
9
User Datagram Protocol(UDP):
An Analogy
•
•
•
•
•
UDP
Single socket to receive
messages
No guarantee of delivery
Not necessarily in-order
delivery
Datagram – independent
packets
Must address each packet
Postal Mail
Postal
Mail
• Single mailbox to receive
letters
messages
• Unreliable 
• Not necessarily in-order
delivery
• Letters
sentisindependently
Each letter
independent
• Must address each reply
Example UDP applications
Multimedia, voice over IP
S’ 10
Lecture 3: Applications
10
Transmission Control Protocol
(TCP): An Analogy
TCP
• Reliable – guarantee
delivery
• Byte stream – in-order
delivery
• Connection-oriented –
single socket per
connection
• Setup connection
followed by data transfer
•
•
•
•
Telephone Call
Guaranteed delivery
In-order delivery
Connection-oriented
Setup connection
followed by
conversation
Example TCP applications
Web, Email, Telnet
S’ 10
Lecture 3: Applications
11
FTP: The File Transfer Protocol
FTP
user
interface
user
at host
FTP
client
file transfer
FTP
server
local file
system
remote file
system
• Transfer file to/from remote host
• Client/server model
• Client: side that initiates transfer (either to/from remote)
• Server: remote host
• ftp: RFC 959
• ftp server: port 21
S’ 10
Lecture 3: Applications
12
Ftp: Separate Control, Data
Connections
• Ftp client contacts ftp server at
port 21, specifying TCP as
transport protocol
• Two parallel TCP connections
opened:
•
Control: exchange commands,
responses between client, server.
FTP
client
“out of band control”
•
TCP control connection
port 21
Data: file data to/from server
TCP data connection
port 20
FTP
server
• Ftp server maintains “state”:
current directory, earlier
authentication
S’ 10
Lecture 3: Applications
13
Ftp Commands, Responses
Sample Commands:
Sample Return Codes
• sent as ASCII text over control
channel
• USER username
• PASS password
• LIST return list of files in
current directory
• RETR filename retrieves
(gets) file
• status code and phrase
• 331 Username OK,
password required
• 125 data connection
already open; transfer
starting
• 425 Can’t open data
connection
• 452 Error writing file
• STOR filename stores (puts)
file onto remote host
S’ 10
Lecture 3: Applications
14
HTTP Basics
• HTTP layered over bidirectional byte stream
• Almost always TCP
• Interaction
• Client sends request to server, followed by response from server to
client
• Requests/responses are encoded in text
• Stateless
• Server maintains no information about past client requests
S’ 10
Lecture 3: Applications
15
How to Mark End of Message?
• Size of message  Content-Length
• Must know size of transfer in advance
• Delimiter  MIME style Content-Type
• Server must “escape” delimiter in content
• Close connection
• Only server can do this
S’ 10
Lecture 3: Applications
16
HTTP Request
S’ 10
Lecture 3: Applications
17
HTTP Request
• Request line
• Method
• GET – return URI
• HEAD – return headers only of GET response
• POST – send data to the server (forms, etc.)
• URI
• E.g. http://www.intel-iris.net/index.html with a proxy
• E.g. /index.html if no proxy
• HTTP version
S’ 10
Lecture 3: Applications
18
HTTP Request
• Request headers
•
•
•
•
•
•
Authorization – authentication info
Acceptable document types/encodings
From – user email
If-Modified-Since
Referrer – what caused this page to be requested
User-Agent – client software
• Blank-line
• Body
S’ 10
Lecture 3: Applications
19
HTTP Request Example
GET / HTTP/1.1
Accept: */*
Accept-Language: en-us
Accept-Encoding: gzip, deflate
User-Agent: Mozilla/4.0 (compatible; MSIE 5.5; Windows NT 5.0)
Host: www.intel-iris.net
Connection: Keep-Alive
S’ 10
Lecture 3: Applications
20
HTTP Response
• Status-line
•
•
HTTP version
3 digit response code
• 1XX – informational
• 2XX – success
•
200 OK
• 3XX – redirection
•
•
•
301 Moved Permanently
303 Moved Temporarily
304 Not Modified
• 4XX – client error
•
404 Not Found
• 5XX – server error
•
•
505 HTTP Version Not Supported
Reason phrase
S’ 10
Lecture 3: Applications
21
HTTP Response
• Headers
•
•
•
•
•
•
•
•
•
Location – for redirection
Server – server software
WWW-Authenticate – request for authentication
Allow – list of methods supported (get, head, etc)
Content-Encoding – E.g x-gzip
Content-Length
Content-Type
Expires
Last-Modified
• Blank-line
• Body
S’ 10
Lecture 3: Applications
22
HTTP Response Example
HTTP/1.1 200 OK
Date: Tue, 27 Mar 2001 03:49:38 GMT
Server: Apache/1.3.14 (Unix) (Red-Hat/Linux) mod_ssl/2.7.1 OpenSSL/0.9.5a
DAV/1.0.2 PHP/4.0.1pl2 mod_perl/1.24
Last-Modified: Mon, 29 Jan 2001 17:54:18 GMT
ETag: "7a11f-10ed-3a75ae4a"
Accept-Ranges: bytes
Content-Length: 4333
Keep-Alive: timeout=15, max=100
Connection: Keep-Alive
Content-Type: text/html
…..
S’ 10
Lecture 3: Applications
23
Cookies: Keeping “state”
Many major Web sites use
cookies
Four components:
1) Cookie header line in the
HTTP response message
2) Cookie header line in HTTP
request message
3) Cookie file kept on user’s
host and managed by user’s
browser
4) Back-end database at Web
site
S’ 10
Example:
•
•
•
Susan accesses Internet
always from same PC
She visits a specific ecommerce site for the first
time
When initial HTTP requests
arrives at site, site creates a
unique ID and creates an
entry in backend database
for ID
Lecture 3: Applications
24
Cookies: Keeping “State”
client
Cookie file
Amazon server
usual http request msg
usual http response +
ebay: 8734
Cookie file
amazon: 1678
ebay: 8734
Set-cookie: 1678
usual http request msg
cookie: 1678
usual http response msg
server
creates ID
1678 for user
cookiespecific
action
one week later:
Cookie file
amazon: 1678
ebay: 8734
S’ 10
usual http request msg
cookie: 1678
usual http response msg
Lecture 3: Applications
cookiespecific
action
25
Typical Workload (Web Pages)
• Multiple (typically small) objects per page
• File sizes
• Why different than request sizes?
• Also heavy-tailed
• Pareto distribution for tail
• Lognormal for body of distribution
• Embedded references
• Number of embedded objects = pareto – p(x) =
akax-(a+1)
S’ 10
Lecture 3: Applications
26
HTTP 1.1 - new features
• Newer versions of HTTP add several new
features (persistent connections, pipelined
transfers) to speed things up.
• Let’s detour into some performance
evaluation and then look at those features
S’ 10
Lecture 3: Applications
27
Packet Delay
• Sum of a number of different delay components.
• Propagation delay on each link.
•
Proportional to the length of the link
• Transmission delay on each link.
•
Proportional to the packet size and 1/link speed
• Processing delay on each router.
•
Depends on the speed of the router
• Queuing delay on each router.
•
Depends on the traffic load and queue size
D B C B A A
S’ 10
Lecture 3: Applications
29
A Word about Units
• What do “Kilo” and “Mega” mean?
• Depends on context
• Storage works in powers of two.
• 1 Byte = 8 bits
• 1 KByte = 1024 Bytes
• 1 MByte = 1024 Kbytes
• Networks work in decimal units.
• Network hardware sends bits, not Bytes
• 1 Kbps = 1000 bits per second
1 Kbit/second
S’•10To avoid confusion,
Lectureuse
3: Applications
30
Application-level Delay
Delay of
one packet
Delay*
Average
sustained
throughput
Size
+
Throughput
Units: seconds +
bits/(bits/seconds)
* For minimum sized packet
S’ 10
Lecture 3: Applications
31
Some Examples
• How long does it take to send a 100 Kbit file?
•
•
Assume a perfect world
And a 10 Kbit file
Throughput
Latency
100 Kbit/s
1 Mbit/s
100 Mbit/s
500 msec
1.0005
0.1005
0.0105
0.1005
0.0006
0.0015
10 msec
1.01
0.11
0.02
0.11
0.0101
0.011
100 msec
0.2
1.1
0.11
0.2
0.1001
0.101
S’ 10
Lecture 3: Applications
32
Sustained Throughput
•
When streaming packets, the network works like a pipeline.
•
•
Throughput is determined by the slowest stage.
•
•
All links forward different packets in parallel
Called the bottleneck link
Does not really matter why the link is slow.
•
•
Low link bandwidth
Many users sharing the link bandwidth
50
S’ 10
37
30
104
59
Lecture 3: Applications
17 267
35
One more detail: TCP
• TCP connections need to be set up
•
“Three Way Handshake”:
Client
Server
SYN (Synchronize)
SYN/ACK (Synchronize + Acknowledgement)
ACK
…Data…
2: TCP transfers start slowly and then ramp up the
bandwidth used (so Lecture
they3:don’t
use too much)
S’ 10
Applications
36
HTTP 0.9/1.0
• One request/response per TCP connection
• Simple to implement
• Disadvantages
• Multiple connection setups  three-way handshake each time
• Several extra round trips added to transfer
• Multiple slow starts
S’ 10
Lecture 3: Applications
37
Single Transfer Example
Client
SYN
0 RTT
Client opens TCP
connection
1 RTT
Client sends HTTP request
for HTML
SYN
DAT
ACK
2 RTT
DAT
FIN
Server reads from
disk
FIN
ACK
3 RTT
Client sends HTTP request
for image
4 RTT
SYN
SYN
ACK
DAT
ACK
S’ 10
ACK
ACK
Client parses HTML
Client opens TCP
connection
Image begins to arrive
Server
Server reads from
disk
DAT
Lecture 3: Applications
38
Performance Issues
• Short transfers are hard on TCP
• Stuck in slow start
• Loss recovery is poor when windows are small
• Lots of extra connections
• Increases server state/processing
• Servers also hang on to connection state
after the connection is closed
• Why must server keep these?
• Tends to be an order of magnitude greater than
# of active connections, why?
S’ 10
Lecture 3: Applications
39
Netscape Solution
• Mosaic (original popular Web browser) fetched
one object at a time!
• Netscape uses multiple concurrent
connections to improve response time
• Different parts of Web page arrive independently
• Can grab more of the network bandwidth than other
users
• Doesn’t necessarily improve response time
• TCP loss recovery ends up being timeout
dominated because windows are small
S’ 10
Lecture 3: Applications
40
Persistent Connection Solution
• Multiplex multiple transfers onto one TCP
connection
• How to identify requests/responses
• Delimiter  Server must examine response for delimiter string
• Content-length and delimiter  Must know size of transfer in
advance
• Block-based transmission  send in multiple length delimited
blocks
• Store-and-forward  wait for entire response and then use
content-length
• Solution  use existing methods and close connection otherwise
S’ 10
Lecture 3: Applications
41
Persistent Connection Solution
Server
Client
0 RTT
Client sends HTTP request
for HTML
DAT
ACK
DAT
1 RTT
Client parses HTML
Client sends HTTP request
for image
Server reads from
disk
ACK
DAT
ACK
DAT
Server reads from
disk
2 RTT
Image begins to arrive
S’ 10
Lecture 3: Applications
42
Remaining Problems
• Serialized transmission
• Much of the useful information in first few bytes
• May be better to get the 1st 1/4 of all images than
one complete image (e.g., progressive JPEG)
• Can “packetize” transfer over TCP
• Could use range requests
• Application specific solution to transport
protocol problems. :(
• Solve the problem at the transport layer
• Could fix TCP so it works well with multiple
simultaneous connections
S’ 10
• More difficult to deploy
Lecture 3: Applications
43
Back to performance
• We examined delay,
• But what about throughput?
• Important factors:
• Link capacity
• Other traffic
S’ 10
Lecture 3: Applications
44
Bandwidth Sharing
• Bandwidth received on the
bottleneck link determines
BW
end-to-end throughput.
• Router before the bottleneck 100
link decides how much
bandwidth each user gets.
•
Users that try to send at a
higher rate will see packet loss
• User bandwidth can fluctuate
quickly as flows are added or
end, or as flows change their
transmit rate.
S’ 10
Lecture 3: Applications
Time
45
Fair Sharing of Bandwidth
• All else being equal, fair
means that users get equal
treatment.
•
Sounds fair
• When things are not equal,
we need a policy that
determines who gets how
much bandwidth.
•
•
•
S’ 10
BW
100
Users who pay more get more
bandwidth
Users with a higher “rank” get
more bandwidth
Certain classes of applications
get priority
Lecture 3: Applications
Time
46
But It is Not that Simple
Bottleneck
S’ 10
Lecture 3: Applications
47
Today
• Application layer
• Each application needs different services
e.g., data loss? Elastic? Timing?
• FTP
• HTTP
• Interaction with TCP: Persistant? Pipelining?
• Delay/Throughput
S’ 10
Lecture 3: Applications
48
Goals [Clark88]
0 Connect existing networks
initially ARPANET and ARPA packet radio network
1. Survivability
ensure communication service even in the presence of
network and router failures
2. Support multiple types of services
3. Must accommodate a variety of networks
4. Allow distributed management
5. Allow host attachment with a low level of effort
6. Be cost effective
7. Allow resource accountability
S’ 10
Lecture 3: Applications
49
Other IP Design Weaknesses
• Weak administration and management tools
• Incremental deployment difficult at times
• Result of no centralized control
• No more “flag” days
S’ 10
Lecture 3: Applications
50
Changes Over Time 
New Principles?
• Developed in simpler times
• Common goals, consistent vision
• With success came multiple goals – examples:
• ISPs must talk to provide connectivity but are fierce
competitors
• Privacy of users vs. government’s need to monitor
• User’s desire to exchange files vs. copyright owners
• Must deal with the tussle between concerns in design
• Provide choice  allow all parties to make choices on
interactions
• Creates competition
• Fear between providers helps shape the tussle
S’ 10
Lecture 3: Applications
51
Summary: Internet Architecture
• Packet-switched
datagram network
• IP is the “compatibility
layer”
• Hourglass architecture
• All hosts and routers run
IP
• Stateless architecture
TCP UDP
IP
Satellite
Ethernet ATM
• no per flow state inside
network
S’ 10
Lecture 3: Applications
53